The pace of the nation's economic expansion slowed sharply during the third quarter to a 2.7 percent annual growth rate, a level some economists said could signal the start of a growth recession.

Although the economy was still far from a traditional recession -- in which the gross national product declines -- several economists said the slowdown raises the specter of a growth recession -- in which growth over two consecutive quarters is too low to create enough jobs to prevent unemployment from rising.

The Commerce Department said it had revised downward its initial estimate for third-quarter GNP from a 3.6 percent rate to 2.7 percent. GNP grew at an annual rate of 10.1 percent in the first quarter and 7.1 percent in the second. Economists said that a rate of growth between 3.0 and 3.5 percent is needed to create enough new jobs to avoid a growth recession.

The unemployment rate has inched down only slowly in recent months as the economy has generated fewer new jobs, and last month stood at 7.4 percent, about the same rate as in May. Last month 270,000 new jobs were created, compared with monthly increases averaging 469,000 from January through May.

Commerce Secretary Malcolm Baldrige downplayed suggestions of a growth recession, asserting that the slowdown "is already behind us" and that growth should pick up later this year and continue into 1985.

Consumer spending picked up last month after a lull this summer, consumer confidence is high and incomes are rising, all of which suggest a resurgence of growth, Baldrige said.

The White House also said growth should remain strong. "The fundamental strength of the economy -- in terms of business investment, personal income, retail sales, low inflation and declining interest rates -- makes it clear that we are on a path of sustainable growth," said White House spokesman Larry Speakes.

The Commerce Department also reported yesterday that inflation as measured by the GNP deflator rose 3.6 percent in the third quarter, compared with 3.3 percent in the second quarter. This measure reflects changes in prices and the composition of output.

Another price indicator that reflects only the change in prices rose 3.8 percent during the third quarter, compared with 4.3 percent in the second quarter. The smaller increase was mainly caused by lower gasoline prices and moderate construction prices, Baldrige said.

The growth in output of goods and services plummeted from the 7.1 percent rate in the second quarter largely because of the continued poor showing in the trade sector and a sustained pause in consumer spending over the summer.

Business fixed investment and government purchases were strong during the third quarter, but personal consumption expenditures and residential construction increased little, Commerce said. Those increases were offset by a sharp decline in net exports, the government said.

But it is consumer spending that accounts for about 60 percent of gross national product. After increasing 7.9 percent in the second quarter, personal consumption expenditures rose just 0.2 percent during the third quarter.

Spending on durable goods declined after a sharp rise in the second quarter. Economists said that consumers apparently have satisfied their demands for goods, and spending probably will increase only as fast as incomes.

In addition, the stimulus from the 25 percent income tax cuts has faded, also suggesting no sharp rises are likely in consumer spending, economists said. However, interest rates are declining, which could encourage spending.

The GNP report "suggests we're flirting with a growth recession," said Allen Sinai, chief economist for Shearson Lehman/American Express. "The third quarter was weak and softer than the numbers suggest. An outright recession has to be ruled out at this time," but not a growth recession.

"It's clear that while we all thought the third quarter would be slow, it's definitely slowing more sharply than I thought," said Lawrence Chimerine, chief economist for Chase Econometrics. "Another number like this next quarter and we'll have a mini-growth recession.

"The key is whether this is just a lull and the economy will continue to grow at very low rates" or whether an outright recession will occur, Chimerine said. "As long as we have these deficits and trade deficits, there will be no good growth on a sustainable basis."

Alan Greenspan, an informal adviser to President Reagan, said that the GNP report suggested "we are starting the fourth quarter with a somewhat stronger rate of growth than the third" quarter, but he said if other economic data are added, "it suggests a less sanguine picture."

Greenspan predicted GNP growth in the fourth quarter would probably be between 3 percent and 4 percent.

Some economists yesterday blamed the sharp slowdown in the economy on the large federal budget deficits, which keep interest rates abnormally high and tend to reduce purchases of homes and big-ticket items.

The high interest rates, coupled with the attractiveness of U.S. assets to foreign investors, are blamed for the high value of the U.S. dollar, which tends to make exports relatively expensive to foreign buyers and worsens the trade balance.

The trade deficit during the third quarter was $22.7 billion, compared with $11.4 billion in the second quarter, Commerce reported.

Economists also said that the success of the economic expansion in the United States and the larger incomes it generated have caused American consumers to purchase more imported goods. At the same time, other countries have not yet recovered enough to afford many U.S. exports, economists said.